Metals play an important role in biocatalysis: more than 20% of all enzymes contain metals in their catalytic centers. One of the most important biometal-catalyzed reactions on earth is the oxidation of water and the release of oxygen catalyzed by the oxygen-evolving complex in Photosystem II. Photosystem II produces all the oxygen in the atmosphere, which humans and animals use for their energy supply by respiration. Photosystem II is a large membrane protein complex embedded in the photosynthetic membrane consisting of 17 proteins to which more than 50 cofactors are bound to perform the complex reactions of light capturing, charge separation and water oxidation. [unreadable] [unreadable] This project has as its main goal the determination of the structure of Photosystem II at atomic resolution and the elucidation of the mechanism of water oxidation by the tetra-nuclear manganese cluster. [unreadable] [unreadable] The physical-chemical parameters of the crystallization process will be determined and the improved crystals will serve as the basis for the determination of the structure of Photosystem II by X-ray crystallography. The electronic structure of the cofactors that are involved in water oxidation will be investigated by spectroscopic investigations using different Electron Paramagnetic Resonance (EPR) techniques on single crystals of Photosystem II. [unreadable] [unreadable] The final goal is to use the structural and functional investigations to unravel the secrets of the mechanism of water oxidation by Photosystem II. [unreadable] [unreadable]